In operations related to the drilling and production of oil and gas from underground reservoirs, sections of tubing which are connected together with fluid-tight reliable seals are used to form a conduit for production of the reservoir fluids (liquid or gas) to the surface. Generally, in the oil and gas industry, threaded connections are often used to connect the tubing sections. Each tubing connection usually has at least one tapered exteriorly-threaded end called a "pin" and a relatively short coupling with two interiorly-threaded ends called "boxes". The pin section is inserted into the box and the "connection" thus made is assembled in a rotational manner, or "made-up", to effectively connect the tubing sections and seal the connection.
The connections (which as described above will include the pin(s) and coupling) for the tubing sections must be manufactured in accordance with certain specifications and tolerances to withstand and carry various loads and to maintain a fluid-tight seal under considerable pressure; the connections will be exposed to torsional forces which arise from making-up or assembling the connection. In addition, the tubing string is usually under pressure resulting from external and/or internal fluids and under axial tension or compression loads caused by the weight of the tubing string hanging in the well. As a result, considerable loads are imposed on the connections between tubing sections which may cause leakage of production fluids from the connections, deformation or galling of the threads of the connections, or separated or split connections.
For relatively moderate depth (less than 10,000 ft), low to moderate pressure (less than 5000 psi) operations, tubing ranging in outer diameter ("OD") from about 1 inch to about 41/2 inches can be used to produce oil and gas from a reservoir; however, the great preponderance of such oilfield applications use tubing having an OD of about 23/8 inches or about 27/8 inches, where the tubing is either non-upset tubing or upset-end tubing. Non-upset tubing is pipe in which the wall thickness and outside diameter in the area of the threads are not increased over the wall thickness and outside diameter of the pipe body, whereas, as described further below, externally upset-end tubing ("upset-end tubing") is pipe in which the wall thickness and outside diameter in the area of the threads are increased. When the threads are cut on the end of the non-upset tubing section, commonly referred to as the pin-end, the cross-sectional area of the tubing body beneath the threads is reduced and hence the axial strength of the connection may be less than that of the tubing body. This reduction in cross-sectional area of the non-upset tubing body is an important factor to be considered when determining what type of thread should be used; however, this strength reduction is not necessarily a hindrance for moderate depth, low to moderate pressure applications.
Connections most often used by the petroleum industry are manufactured in accordance with specifications and tolerances provided by the American Petroleum Institute. These are referred to as "API" connections. For non-upset tubing having an OD of about 23/8 inches or about 27/8 inches, the existing API specification teaches use of a "10-round" (ten rounded threads per linear inch) threaded connection design, which is a relatively fine and shallow-cut thread. This thread is often too fragile for oilfield handling and is not a popular choice due to its propensity to cross-thread and otherwise become damaged during make-up and break-out operations. As a consequence, for most oil field operations, non-upset tubing having an OD of about 23/8 inches or about 27/8 inches with 10-round threads is seldom used. A typical 10-round thread is dimensionally described in Table 2.9 of API specification STD 5B (thirteenth edition, May 31, 1988).
Because of the problems associated with the 10-round thread connection for non-upset tubing having an OD of about 23/8 inches or about 27/8 inches, an upset-end tubing connection is normally used instead. Generally, with an upset-end tubing connection, to maintain a connection at least as strong as the tubing body, the OD of one end of the tubing is increased by thermally and mechanically upsetting the tube end, which increases the cross-sectional wall thickness and also the outer diameter of the tube end in preparation for thread cutting. Because of the increased wall thickness, the axial strength of the connection is typically greater than that of the tubing body.
The existing API specification (STD 5B) for upset-end tubing connections with tubing having an OD of about 23/8 inches or about 27/8 inches teaches use of an "8-round" thread form (eight rounded threads per linear inch), which is a taller thread and is more sturdy than the 10-round thread and is well accepted in the oil field due to its rugged characteristics. However, the API specifications do not teach the use of an 8-round thread form with non-upset tubing having an OD of about 23/8 inches or about 27/8 inches.
Although the API Specification (STD 5B) teaches the use of the rugged 8-round thread form with upset-end tubing, there are problems associated with the upsetting process; it is complex and time-consuming and furthermore requires subsequent heat treatment to restore the upset region to its original strength and ductility. The upsetting operation can introduce thermal cracks and internal irregularities which can render subsequent internal coatings ineffective against. corrosion and even lead to downhole failures. The upsetting operation requires an extra inspection process just to cull out the thermally-cracked tubulars. These additional processing steps and precautions, which are not required for non-upset tubing, can add at least 10% to the total cost of the tubing. Furthermore, when upsetting welded tubulars (i.e., tubulars which are created by rolling a steel plate into a circle and welding longitudinally), the upsetting process becomes even more complex and expensive because additional precautions must be taken. Welded tubulars are usually less expensive than seamless pipe, yet the additional precautions which must be taken when upsetting welded tubulars will often offset these cost advantages. Accordingly, if a connection design does not require the upsetting process (which, as set forth above, can add at least 10% to the total cost of the tubing), tubular costs could be reduced another 10% to 30% by using seam-annealed, electric resistance welded (ERW) pipe rather than seamless pipe. In addition to the foregoing problems related to the upsetting operation, an upset tubing end can only be rethreaded two or three times because of the limited length of the upset-end, whereas a non-upset tubing section can be rethreaded 10 or 20 times until the tubing is too short to be useful.
In view of the foregoing, there is a need for rugged low-cost tubing connections for use with non-upset tubing having an outer diameter of about 23/8 inches or about 27/8 inches.